Lately, the number of wind generators and wind farms connected to the power grid is dramatically growing. For this reason, grid connection requirements are becoming increasingly demanding and, as a consequence, wind generator control systems become more complex each year. Specifically, grid connection requirements require the wind generator to perform well during short duration voltage drops in the power grid.
A back-to-back converter is a known device employed for connecting a generator to the power grid. A back-to-back converter comprises a generator side converter and a power grid side converter connected by means of a DC-link. Back-to-back converters are employed in DFIG (doubly fed induction generator) systems, in which case they connect the rotor of the generator to the power grid, and also in full converter systems, in which case they connect the stator or the generator to the power grid. Both DFIG and full converter systems are part of the prior art.
More specifically, DFIG is nowadays widely employed due to the fact that it provides a number of economic and technical advantages over most other systems. In a DFIG, if no action is taken, a voltage drop in the power grid produces a transient phase causing an overvoltage in the rotor windings which may be destructive for the generator side converter.
Therefore, in order to fulfil the grid connection requirements, DFIG systems include a device allowing, on one hand, to keep the generator connected to the grid during the transient phase, and, on the other hand, to maintain control of the wind turbine. This device is typically known as “crowbar” or “crowbar unit”. International patent application PCT/ES2006/000264 discloses a crowbar unit which is activated when a voltage drop in the power grid is detected. The crowbar unit lowers the voltage at the rotor windings basically by means of a short-circuit, therefore protecting the generator side converter of the back-to-back converter.
Recently, increasingly demanding grid connection requirements require a shorter transient phase. However, if current crowbar units are prematurely deactivated, destructive voltages may still be present in the rotor windings. Therefore, a need still exists of control systems for electric generators connected to the power grid.